The present invention generally relates to semiconductor devices and more particularly to a fabrication of a semiconductor device and a semiconductor integrated circuit device that has an interlayer insulation film of low permittivity.
In a semiconductor integrated circuit in which a number of semiconductor devices are integrated monolithically, each of the semiconductor devices are covered by an interlayer insulation film. In the art of semiconductor integrated circuit, it is commonly practiced to provide an interconnection pattern further on such an interlayer insulation film to form a multilayer interconnection structure. In recent miniaturized semiconductor integrated circuits, such interconnection patterns are also miniaturized. In relation to this, there emerges a problem in that the electric signals experience a delay due to the permittivity of the interlayer insulation film.
FIGS. 1A-1C show a conventional process for forming a multilayer interconnection structure.
Referring to FIG. 1A, an SiO.sub.2 film 13 is formed on a substrate 11 by an ordinary plasma CVD apparatus of the parallel-plate type, such that the SiO.sub.2 film 13 covers an interconnection pattern 12 formed on the substrate 11. The SiO.sub.2 film 13 thus formed, while having an advantageous feature of low leakage current, generally suffers from the problem of poor step coverage. Thus, it is practiced to deposit another SiO.sub.2 film 14 on the structure of FIG. 1A under a high-density plasma environment while applying a substrate bias, as indicated in FIG. 1B. As the SiO.sub.2 film 14 is deposited under existence of the substrate bias, the SiO.sub.2 film 14 experiences a sputter etching simultaneously to the deposition by the high-density plasma formed in a reaction chamber. As a result of such a sputter etching that competes with the deposition, the SiO.sub.2 film 14 shows an excellent step coverage.
After the SiO.sub.2 film 14 is thus formed, a chemical mechanical polishing process (CMP) is applied to the SiO.sub.2 film 14 for a planarization thereof as indicated in FIG. 1C.
In the planarized structure thus obtained, however, there arises a problem that the operational speed of the semiconductor device does not increase as is expected from the miniaturization, due to the fact that the interlayer insulation film 13 or 14 has a relatively large permittivity, as noted already.
Meanwhile, it is known that the permittivity can be reduced in an insulation film including therein an Si--O bond such as SiO.sub.2, by introducing F (fluorine) into the insulation film. The interlayer insulation film 13 or 14 also shows a reduced permittivity with increasing F content in the film. Thus, there is a prospect that a maximization is possible for the operational speed of the semiconductor device by introducing F atoms into the insulation film.
However, conventional attempts to incorporate F atoms into the interlayer insulation film, conducted in a conventional parallel-plate type plasma CVD apparatus, although have been successful at reducing the initial permittivity, have failed due to the problem of unwanted increase of hygroscopicity. As a result of increased hygroscopicity, the permittivity of the interlayer insulation film increases with time, and the desired effect of the decrease of the initial permittivity is canceled out ultimately. Further, the interlayer insulation film, after absorbing moisture in the air, has a poor film quality, and various problems such as corrosion or peeling of the interconnection pattern are caused.
While using a conventional parallel-plate type CVD apparatus, it has not been possible to incorporate a sufficient amount of F atoms into the interlayer insulation film while simultaneously maintaining high film quality. Thus, the permittivity of the film could not be reduced below about 3.8.
On the other hand, it is possible to improve the quality of the interlayer insulation film to a certain extent by using a high-density plasma formed by a induction coupling process (ICP) or electron cyclotron resonance (ECR). However, even such an interlayer insulation film formed under the high-density plasma environment shows an increased hygroscopicity when the F content in the insulation film is increased, and the permittivity cannot be reduced below about 3.5. For example, the interlayer insulation film 13 or 14 formed by the process of FIGS. 1A-1C shows a poor film quality due to the hygroscopicity even when the film formation is made under a high-density plasma condition.
Further, such an interlayer insulation film doped with F tends to cause a problem of corrosion when formed directly on a metal pattern such as an Al interconnection pattern. Further, one encounters a problem, when covering a line-and-space pattern by such an interlayer insulation film, such that the part of the interlayer insulation film filling a gap between adjacent line patterns is decomposed by the action of excessive F in the interlayer insulation film. When this occurs, the step coverage of the line-and-space pattern by the interlayer insulation film is deteriorated substantially.
Meanwhile, it is known conventionally that the permittivity of an interlayer insulation film can be reduced to 2.0-3.0 by using a spin-coated SOG (spin-on-glass) for the interlayer insulation film. However, such an SOG film tends to create a tensile stress field on the substrate surface, and the tensile stress field thus created tends to induce a warp in the substrate such that the substrate surface forms a concaved surface. When such a warp is formed, there occurs various problems such as distortion or displacement of various patterns to be formed on the interlayer insulation film during the photolithographic patterning process.
In order to eliminate this problem, it has been practiced to provide a cap layer of F-doped SiO.sub.2 on the interlayer insulation film by using a parallel-plate plasma CVD process or a high-density plasma CVD process. However, the permittivity that can be achieved for such a cap layer by the parallel-plate plasma CVD process is about 3.8 at best. Further, the attempt for further reduction of the permittivity tends to cause the problem of unwanted degradation of the hygroscopicity of the film. Even when the high-density plasma CVD process is used, the permittivity of the obtained cap layer cannot be reduced below about 3.5.